Polyolefins have a drawback that they have a lower adhesive property, a lower printing property and a lower compatibility with other polymers due to having no polar groups. To overcome the drawback, copolymerization of an olefin with a polar monomer has been attempted. In addition, it is known that an olefin may be copolymerized with methyl methacrylate for the purpose of improvement in an impact resistance, improvement in a flexibility, reduction of a water absorption and imparting a surface hydrophobicity.
Processes for preparing these copolymers have been disclosed; for example, JP-A 59-43003 or JP-A 64-14217 which has disclosed a process for preparation of a copolymer using a titanium catalyst or a nickel catalyst, respectively. Both processes have a drawback that they require a large amount of a Lewis acid or alumioxane as a co-catalyst.
JP-A 1-201302, JP-A 1-201304 and JP-A 63-43914 have disclosed processes that a polyolefin is modified at its ends and is then block-copolymerized with a (meth)acrylate.
However these processes have a drawback that an introduction ratio of the modified ends is low, so that homopolymers of the (meth)acrylate may be produced as by-products.
Furthermore, JP-A 3-255116 and JP-A 4-53813 have disclosed processes that a block copolymer of ethylene-(meth)acrylate may be produced using a monofunctional, trivalent, rare earth compound. However, the former process does not provide a copolymer containing (meth)acrylate in a high proportion, while the latter process provide products with a wide distribution of molecular weight, i.e., Mw/Mn&gt;2, due to a chain-transfer reaction during the polymerization of ethylene and gives homopolymers of ethylene as byproducts. In addition, since ethylene does not react with a propagating end of (meth)acrylate, these processes can provide only two-component block copolymers of ethylene-(meth)acrylate.
Recently, difunctional rare earth complexes as a polymerization initiator have been proposed for preparation of homopolymers of ethylene or copolymer of ethylene with other monomers. Specifically, William J. Evans et al. have polymerized ethylene using a difunctional rare earth complex as an initiator (J.Am.Chem.Soc., 1990, 112, pp.2314-2324). Bruce M. Novak and Lisa S. Boffa have obtained a three-component block copolymer of poly(methyl methacrylate) block-poly(ethyl acrylate) block-poly(methyl methacrylate) block, using a difunctional rare earth complex as an initiator (ACS Polymer Preprint., 35(2), 1994 and Boffa, MACRO AKRON '94, 117). Yasuda et al. have polymerized ethylene using a bivalent samarium complex as an initiator. (Polymer Preprints). Japan, Vol.43, No.6(1994)). These reports, however, do not imply an A-B-A type of block copolymer of this invention.
Generally, as described in "P. L. Watoson and T. Herskorvitz ACS symposium series No.212, p.459-479", it is known that a bivalent samarium complex makes ethylene polymerize according to the following formula (III).
Specifically, when both ends of a polymer produced are living ends, an A-B-A type of block copolymer may be produced by adding another monomer. ##STR1##
The present inventors have proposed the preparation of an A-B-A type of block copolymer consisting of ethylene and a (meth)acrylate in JP-A 6-306112 by applying the above concept, but the process has a drawback that a copolymer produced has a Mw/Mn of 4 to 6, indicating a wide distribution of molecular weight, and it produces a large amount of polymers as byproducts such as diblock copolymers and homopolymers of methyl methacrylate.
We have intensively attempted to solve the problems and finally discovered that a particular initiator may provide a three-component block copolymer of (meth)acrylate--olefin--(meth)acrylate having a narrow distribution of molecular weight, to achieve this invention.